Tachykinins are a group of naturally occurring peptides found widely distributed throughout mammals, both within the central nervous system and in the peripheral nervous and circulatory systems. The three known mammalian tachykinins are Neurokinin-1 (NK-1, substance P), Neurokinin A, and Neurokinin B. These compounds act as neurotransmitters and immunomodulators and may contribute to the pathophysiology of a wide variety of human diseases.
Receptors for tachykinins have been identified and include neurokinin-1 (NK-1 or Substance P-preferring), NK-2 (Neurokinin A-preferring) and NK-3 (Neurokinin B-preferring). NK-1 receptor antagonists are being developed for the treatment of physiological conditions associated with an excess or imbalance of tachykinins, particularly substance P. Such conditions include affective disorders such as anxiety, depression, obsessive compulsive disorder, bulimia, and panic disorder. See Gentsch et al. Behav. Brain Res. 2002, 133, 363; Varty et al. Neuropsychopharmacology 2002, 27, 371; Papp et al. Behav. Brain Res. 2000, 115, 19; Kramer et al. Science 1998, 281, 1640; and Rosen et al. Bioorg. Med. Chem. Lett. 1998, 8, 281. Robust antidepressant activity has been reported for two NK-1 antagonists, MK-869 (M. S. Kramer, et al., Science 1998, 281 1640) and CP-122,721 (T. J. Rosen, et al., Bioorganic and Medicinal Chemistry Letters 1998, 8, 28 and CNS Drug News, December, 2000, 24).
Selective serotonin reuptake inhibitors (SSRI's) have proven to be effective in treating depression, but have the disadvantages of delayed onset of antidepressant activity, limited efficacy, and significant side effects. See Novel strategies for pharmacotherapy of depression, K. A. Maubach, N. M. J. Rupniak, M. S. Kramer, and R. G. Hill, Current Opinion in Chemical Biology 1999, 3, 491-499. Selective serotonin reuptake inhibitors (SSRIs) in combination with other agents can be useful for the treatment of depression and other disorders and combination SERT/NK1 compounds should also be useful for these conditions. For example, the combination of SSRIs with dopamine reuptake inhibitors such bupropion and modafanil have shown clinical benefit relative to SSRIs alone, primarily due to superior side effect profiles (Bodkin et al, 1997, J Clin Psychiatry, 58: 137-145; Kennedy et al, 2002, J Clin Psychiatry, 63:181-186). Additionally, the combination of SSRIs with 5-HT1A antagonists such as pindolol have shown improved clinical response relative to SSRIs alone (Artigas F et al, 1994, Arch Gen Psychiatry 51:248-251; Blier P and Bergeron R, 1995, J Clin Psychopharmacol 15:217-222). Finally, combining SSRIs with antipsychotics, such as fluoxetine plus olanzapine, has provided superior profiles in certain depressed populations including psychotic depression and bipolar depression (Corya et al, 2003, J Clin Psychiatry, 64:1349-1356; Rothschild et al, 2004, J Clin Psychopharmacol, 24:365-373).
NK-1 antagonists are believed to modulate 5-HT function via noradrenergic pathways and have been shown to attenuate presynaptic 5-HT1A receptor function. NK-1 antagonists offer an alternative approach for treating depression in patients that respond poorly to the SSRI's and other available drugs and the combination of serotonin reuptake inhibition with NK-1 antagonism may lead to new classes of drugs with improved characteristics.